Transesterification of a simple ester compound with a simple alcohol compound is known to occur under basic conditions. The transesterification reaction is an equilibrium reaction which can be driven to completion by removing the alcohol moiety evolving from the cleaved ester. If the cleaved alcohol moiety is a low molecular weight lower alkyl alcohol such as methanol or ethanol, removal by evaporation is quite easy. It has been found that transesterification as a curing mechanism for crosslinking polymers used in paint coatings provides an attractive cure mechanism for producing thermosetting protective coatings since cleaved lower alkyl alcohols can be easily removed from the coating by simple air dry evaporation thereby driving the transesterification reaction to completion.
Related applications are as follows:
Commonly assigned Ser. No. 871,444 filed June 6, 1986, now being U.S. 4,749,728 discloses hydroxyl functional polymers adapted to cure by transesterification with a carboxylic ester functional polymer in the presence of transesterification catalysts. The transesterification catalysts comprise an onium salt or a Lewis base in combination with a catalytic amount of epoxy compound.
Commonly assigned Ser. No. 138,149 filed Dec. 28, 1987 pertains to similar polymers adapted to cure by transesterification in the presence of transesterification catalysts comprising a conjugate base of a weak acid having pka between 2.5 and 14 in combination with a catalytic amount of epoxy compound.
Commonly assigned Serial No. 157,249 filed Feb. 18, 1988 pertains to similar polymers adapted to be cured by transesterification in the presence of transesterification catalysts comprising an inorganic salt selected from halogens, azides,cyananides, and hydroxides in combination with a catalytic amount of epoxy compound.
Commonly assigned Ser. No. 251,762 filed concurrently Oct. 3, 1988 (Doc. 10328) pertains to similar coatings adapted to crosslink by transesterification based on polyester polymer containing a beta-hydroxy ester group activated by certain transesterification catalysts to crosslink with hydroxyl groups on polymers.
Commonly assigned Ser. No. 242,817 filed concurrently on Sept. 12, 1988 (Doc. 10332) pertains to similar coatings adapted to crosslink by transesterification based on a self-curing acrylic polymer containing beta-hydroxy ester groups adapted to coreact upon being activated by certain transesterification catalysts.
It now has been found that thermosetting paint coatings can be further improved based on thermosetting polymers comprising a hydroxyl functional polyester polymer adapted to be crosslinked with a beta-hydroxy ester acrylic copolymer by a transesterification cure in the presence of certain catalyst combinations. In accordance with this invention, beta-hydroxy ester acrylic copolymer crosslinkers are utilized instead of simple methyl ester polymers to produce considerably improved cured film integrity properties of paint films.
With respect to prior art, several patents disclose the use of acids, bases, metal salts, and organic metal complexes as catalysts for transesterifying polymers such as U.S. 4,362,847; U.S. 4,376,848; U.S. 4,332,711; and U.S. 4,459,393 wherein octoates or naphthenates of lead, zinc, calcium, barium, and iron are disclosed as transesterification catalysts.
Green U.S. 4,559,180 discloses an organic syntheses process for simple transesterification of very low molecular weight simple carboxyl ester compounds with similar simpler alcohols by reacting the same in the presence of an epoxide and a Lewis base (containing a Group V element) or a cyclic amidine. The Green patent does not pertain to coatings or polymers but merely discloses simple chemical reactions between very low molecular weight chemical compounds.
Dante and Parry have shown that phosphonium halides, such as ethyltriphenyl phosphonium iodide, are efficient catalysts for (a) 1,2-epoxide reactions with phenols to yield hydroxyl ethers (U.S. 3,477,990), and (b) polyepoxide reactions with carboxylic acids or acid anhydrides (U.S. 3,547,885). The patents suggest that polyepoxides and phenols can be reacted to form phenolic hydroxyl ethers with phosphonium salts as catalysts. The counterion of the phosphonium moiety is the anion portion of a carboxylic acid, or acid ester, such as in ethyltriphenyl phosphonium acetate (U.S. 3,948,855).
Barnhoorn et al (U.S. 4,459,393) teach self-crosslinking thermosetting resin compositions obtained from the reaction of a beta-hydroxyalkyl ester of an alpha,beta-carboxylic acid with a primary mono- or polyamine to give a product having 1 to 2 amino hydrogens and further reacted with a polyglycidyl ether of a polyhydric phenol so that the final resin adduct has more than one beta-hydroxyalkyl ester group and amine groups having 1 to 2 amine hydrogen atoms per molecule. Transesterification catalysts known in the art are taught.
Subramanyam et al (U.S. 4,376,848) teach the preparation of water dilutable electrocoating compositions having tertiary amino-containing basic binders by reacting a secondary amino group compound with an olefinically double-bonded epoxy and the copolymerization of this product with at least one ethylenically bonded polymerizable monomer wherein said binders can self-cure and be cured in combination with amine resins and/or phenolic resins. Common transesterification catalysts are taught.
Velko patents disclose resinous compositions curable through a transesterification curing mechanism based on conventional heavy metal catalysts. For instance, U.S. 4,423,167 discloses a polymeric polyol adapted to be crosslinked with a polyester having at least two beta-alkoxyester groups in the presence of conventional transesterification catalysts. Similarly, U.S. 4,489,182 is based on a crosslinking agent having at least two delta-hydroxy ester groups, while U.S. 4,423,169 is based on a crosslinking agent having at least two beta- and/or gamma-ester groups, and U.S. 4,423,168 is based on a crosslinking agent having at least two beta-amide ester groups. The Valko patents utilize as catalysts organic salts of conventional heavy metal catalysts such as lead, zinc, iron, tin and manganese.
In accordance with this invention, highly desirable thermosetting paint coatings are based on a polymeric binder comprising a hydroxyl functional polymer, preferably a polyester polymer, adapted to be crosslinked by transesterification with a beta-hydroxy ester addition copolymer, preferably an acrylic copolymer, upon being activated with a catalyst comprising certain nucleophiles in combination with a catalytic amount of epoxy compound. The novel transesterification cure using beta-hydroxy esters instead of simple methylesters as the crosslinking component provides a significant improvement in film properties. The beta-hydroxy crosslinking system of this invention is particularly useful in powder coatings, high solids liquid coatings, appliance coatings and thermosetting coatings generally. A particularly good balance of hardness and flexibility properties in film integrity characteristics can be achieved which is particularly important in a variety of industrial coating applications. Films with exceptional hardness and impact resistance can be prepared, for example, by blending as little as 5% hydroxyl functional polyesters into acrylics. Cure of the acrylic without the added polyester gave films with good hardness, but less than satisfactory flexibility. Epoxy nucleophile catalyzed transesterification in combination with the polyester polymer and acrylic copolymer provide unique combination of properties.
Acrylic films have desirable properties, such as weatherability, moisture resistance, detergent resistance, etc., but generally do not provide good impact resistance. Polyester films can have good impact resistance with good hardness, but they lack many of the good properties of acrylics. When blended and cured by conventional crosslinking chemistry, polyesters and acrylics can produce films which compromise quality. At a 50/50 ratio of polymers with melamine crosslinking, a polyester/acrylic blend will have fair impact resistance of 50 to 100 inch pounds at H to 2H hardness, but impact decreases rapidly as the proportion of acrylic increases, and other desirable properties, such as weathering, fall off rapidly as the proportion of polyester is increased.
Contrary to the foregoing accepted rule of compromise, it now has been found that acrylics can be cured by transesterification in accordance with this invention with a very small level of a variety of preferred polyesters at levels as low as about 5%. Resulting films will have hardnesses to 4H with impact resistances of 160 inch pounds of impact, forward and reverse. This discovery provides a means of formulating industrial paints with a unique mix of properties, including water, detergent, and weathering resistance, while maintaining hardness and impact resistance.
These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.